1. Field of the Invention
This invention relates to signal switching apparatus, and particularly, to cross-point or matrix type switches.
2. Description of the Prior Art
Present methods of controlling the interconnection electrical signal lines from one set of equipment to another include patch-boards, manually operated mechanical switches, electrically operated cross-bar switches, electrically operated relays, electrically operated rotary switches, electrically operated matrices of reed relays, and electronically operated semiconductor switches. The best known example of automatic switching systems are those used in the modern telephone system. A telephone central office contains a network of switching devices by means of which subscriber lines and trunks can be interconnected. Telephone line switching is usually accomplished with electromechanical relays, including reed relays, electromechanical crossbar switches, or, in some cases, semiconductor switches. The Bell System Electronic Switching System (ESS) is representative of the present state of the art in telecommunications switching systems. The ESS system uses reed type relays, where the contacts are mounted on thin strips or reeds of ferromagnetic material. The reeds and their contacts are encapsulated in a glass envelope and are controlled by an external magnetic coil. Operate and release times are in the 2 to 20 millisecond range. The reed relays are usually configured in an array termed a cross-point switch. Two reed relays are normally required for each cross-point. A 256 .times. 256 cross-point switch requires 65,536 cross-points and 131,072 reed relays.
Semiconductor electronic switches are now available to a limited degree. Such switches usually utilize metal oxide semiconductor (MOS) or silicon controlled rectifier (SCR) technology. Both MOS and SCR type switches are available as 4 .times. 4 cross-point arrays on a single chip. Larger arrays require suitable interconnection of the 4 .times. 4 arrays. A straightforward 256 .times. 256 cross-point switch would require 4,096 of the 4 .times. 4 arrays. Typical ON resistance of such switches is 10 to 100 ohms, with switching times of 0.2 to 3.0 microseconds.
It is the object of this invention to provide a novel and improved cross-point type switch which can handle the interconnection of large numbers of input and output signal lines with very high speed dynamic switching.
It is a further object to provide a cross-point switch which can be easily controlled by remote apparatus, such as a computer.
It is a further object of this invention to provide a switching apparatus which can handle on the order of 256 input lines and 256 output lines in a single level of switching and in such a way that the signal on any input line can be connected to any output line in any order, once the addresses of the input and output lines are received by the switching apparatus.
It is a further object of the invention to provide a switching means which does not depend upon mechanical contacts to make connections.
In many applications, it is desirable to isolate the inputs from the outputs in such a way that they have no electrical connections. Therefore, it is a further object of this invention to provide optical isolation of inputs from outputs.
Another object of this invention is to provide an inexpensive and reliable switch which can handle several hundred input and output signal lines in a simple, single level implementation.
In some applications, the signals to be switched are analog in nature, i.e., continuous, such as audio, while in other applications the signals are digital in nature, i.e., quantized to two levels, usually termed "one" and "zero". It is therefore an object of this invention to provide a switch which will handle either analog or digital signals.
In some applications, it is desirable to utilize a space-division switch in a time-sharing mode in which each input is time-shared with two or more signal sources. It is therefore an object of this invention to provide a space-division switch with very high switching speed, on the order of 100 nanoseconds, so that inputs may be time-shared with multiple signal sources.